Method and arrangement for plasma boronizing

ABSTRACT

A method and arrangement for producing a boride layer on a surface by plasma boronizing includes supplying a gas mixture containing a boron-releasing gas to a reactor and generating a glow discharge in the reactor using a pulsed DC voltage. The parameters of the production of the plasma produced by the glow discharge in a treatment chamber of the reactor are selected so that an increased quantity of excited boron particles is generated in the plasma to produce non-porous boride layers, for example, for boride coating of components which need a surface that is highly resistant to wear, for example, gears, camshafts and the like. Parameters with which the production of the boride layer can be controlled are, for example, voltage, pulse-duty factor, frequency, temperature, treatment chamber pressure during the production of the plasma, and the content of boron-releasing gas and of the remaining components in the gas mixture which is fed to the reactor.

REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No.PCT/EP98/08079 filed Dec. 11, 1998.

BACKGROUND OF THE INVENTION

This invention relates to methods and arrangements for producing aboride layer on a surface by plasma boronizing in which a gas mixturecontaining a boron-releasing gas is supplied to a reactor in which aglow discharge is generated.

The boronizing process, which belongs to the group of thermochemicalmethods of treatment, makes it possible to produce wear-resistantsurface layers, preferably on metal components, which provide excellentprotection against high abrasive and adhesive wear stresses. Until now,industrial boronizing processes have frequently used solidboron-releasing media in the form of, for example, powders or pastes.However, such processes have a number of drawbacks which limit theproduction of borides to certain applications for which no alternativetreatments that would provide a comparable wear resistance exist. Thesedrawbacks include, for example, the high cost of manual handling of thematerials used. In this regard, the component to be boronized must bepacked in boron-releasing powder or the boronizing paste must be spreadon the component, and the residual boronizing agent must be removedafter completion of the boronizing. For ecological reasons, all residuesof boronizing agent must be disposed of at suitable waste-disposaldumps. Frequently, the prior-art methods cannot be adequately controlledor cannot be controlled at all and automation of such processes isimpossible.

For this reason, various methods have been developed for producing aboride layer on a surface by plasma boronizing in which a gas mixturecontaining a boron-releasing gas is supplied to a reactor and a glowdischarge is generated within the reactor. Such a process is described,for example, in German Offenlegungsschrift No. 196 02 639. Thatpublication discusses the problem of plasma boronizing of metalsurfaces, for example, in which the resulting layers have a substantialporosity. Such porosity has a negative effect on the wear resistance ofthe boronized surface. Moreover, the plasma boronizing method asdescribed in the aforementioned publication cannot be developed forindustrial series applications.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod and arrangement for plasma boronizing which overcomesdisadvantages of the prior art.

Another object of the invention is to provide a plasma boronizing methodwhich produces substantially pore-free boronized surfaces in a reliablemanner and is therefore suitable for industrial series applications.

These and other objects of the invention are attained by providing aboronizing method in which a gas mixture containing a boron-releasinggas is supplied to a reactor in which the gas releases boron in a glowdischarge plasma and a glow discharge parameter is controlled tomaintain the amount of at least one glow discharge product of theboron-releasing gas, or the relation of that amount to the amount ofanother glow discharge product, within selected maximum and minimumlimits, or in which the glow discharge pulse ratio is maintained above aselected level, or the glow discharge pulse duration is maintained belowa maximum value.

The invention further provides an arrangement for producing a boridelayer on a surface by plasma boronizing including a reactor having atreatment chamber to which a gas mixture containing a boron-releasinggas is supplied and in which a glow discharge is generated by a DCvoltage having a variable pulse width or pulse pause. This arrangementis suitable for carrying out the method of the invention in accordancewith the abovementioned parameters, and will be described in detailhereinafter.

First, various alternatives for carrying out the method according to theinvention will be described in greater detail. It has now beenestablished by extensive testing that it is important in plasmaboronizing to properly select the production parameters of the plasmagenerated in the treatment chamber of the reactor. It has surprisinglybeen found that these parameters can be selected advantageously in sucha way that an increased proportion of excited boron particles isproduced in the plasma. If the plasma contains a relatively large amountof excited boron, boron layers of low porosity will be produced. Thisfact was demonstrated by optical emission spectroscopy and plasmaanalysis during development work leading to the method of the invention.If, on the other hand, the plasma contains a high concentration ofexcited BC1 particles, highly porous layers are produced which should beavoided for the abovementioned reasons. During the studies whichresulted in the invention, the inventors established that variousparameters with respect to both plasma generation and the individualcomponents contained in the gas mixture supplies to the reactor can havean effect on the desired concentration of excited boron particles. It isimportant that, in order to obtain the desired boride layer of lowporosity, certain threshold values of excited boron be attained in theplasma.

In accordance with one embodiment of the plasma boronizing method of theinvention, a glow discharge is preferably generated with a pulsed DCvoltage. In this connection, it was found surprisingly that control ofthe pulse-duty factor, which is defined as the ratio of the duration ofthe voltage pulse to the subsequent pulse pause before the next voltagepulse, facilitates the desired production of an increased concentrationof excited boron particles and thus facilitates optimum control of theplasma generation method. According to one variation of the method ofthe invention, the pulse-duty factor should be greater than 1.1:1,preferably in the range from 1.25:1 to 5:1, and desirably in the rangefrom 1.5:1 to 3.5:1. Furthermore, the pulse period, which is the sum ofthe durations of the voltage pulse and pulse pause, is preferably lessthan about 230 μs, but ≧50 μs.

Desirably, the pulse period is less than about 230 μs and more than 50μs, for example, about 210 μs. According to one embodiment of the methodof the invention, the DC voltage used for the pulsed current to producethe glow discharge is preferably in the range between about 500 voltsand about 1000 volts, desirably in the range between about 600 volts andabout 900 volts, and more desirably in the range between about 650 voltsand about 800 volts. It was further found that, when working with highervoltage, the use of a longer pulse pause is advantageous. However, agood result is also achieved when applying lower voltage, preferablywithin the abovementioned voltage ranges, but even in this case, thecomposition of the individual components of the gas mixture supplied tothe reactor can exert an influence on the resulting coatingcharacteristics.

In the method of the invention, it is preferable to use, as a firstcomponent of the gas mixture supplied to the reactor, a boron-releasinggas in the form of a boron halide, for example, boron trichloride orboron trifluoride. Preferably used as second component of the gasmixture is hydrogen gas, and optionally, as third component of the gasmixture, a noble gas such as argon. It has been found that, when usingargon as a third component in the method of the invention, good boridelayers can be obtained even when applying relatively low voltages.

The concentration of boron trihalide as a boron-releasing gas in the gasmixture generally has an effect on the boride layer produced by themethod. In general, the boron trihalide content should not be too lowand, as rule, should not be less than 1% by volume, since otherwise asuitable boride layer may not be obtained. In accordance with oneembodiment of the method of the invention, the boron trihalide contentis preferably in the range of about 2% by volume to 50% by volume. Itshould be noted, however, that excessively high boron trihalide contentscause a relatively high boron trihalide loss. The lost boron trihalideis contained in the waste gas from the reactor and results in high costfor waste gas disposal or purification. Especially good results havebeen obtained with the process of the invention when a boron trihalidecontent preferably in the range between about 2% by volume and 10% byvolume, for example, about 7.5% by volume of boron trihalide, is used.If a noble gas is used as a third gas mixture component in the method ofthe invention, then the content of the noble gas, for example, argon, ispreferably in the range between about 0% by volume and about 20% byvolume. The second component of the mixture is preferably hydrogen gasin an amount corresponding to the remainder of the gas mixture contentusing the above-specified preferred ranges of the other two components,i.e. the boron trihalide and the noble gas.

The method according to the invention is preferably carried out at a lowpressure, for example, a pressure in the range between about 0.5 andabout 15 hPa, desirably in a range between about 1 and 10 hPa.

The selection of the desired method parameters for achieving the desiredcoating characteristics can be achieved, for example, in such a way thatthe amount of excited boron particles in the plasma is determinedanalytically and one or more process parameters are changedappropriately to generate the glow discharge parameters such as voltage,pulse duty factor, frequency, temperature, pressure, etc., which producethe desired result.

According to one embodiment of the method of the invention, the boridelayer can be produced in several stages, in which case, for example, thecoating is carried out at a relatively low treatment temperature duringa first stage in order to reduce halide formation in the plasma which ispartially responsible for pore formation. In this first stage, arelatively thin but pore-free boride layer is produced which is moreresistant to corrosive attack. Subsequently, in a second stage, thetreatment temperature is raised in order to enhance the diffusion of theboron particles and thus the formation of a layer of increasingthickness. Even when a parameter such as, in this case, the treatmenttemperature, is changed in such a two-stage or optionally multistageprocess, care should be taken that the other process parameters arecontrolled so that, if possible, an increased content of excited boronparticles is obtained in the plasma in order to enhance the boroncoating reaction and avoid a corrosive attack.

It has been found that, in the method of the invention, adjustment ofthe current passing through the plasma generally has a considerableeffect. The influence exerted on the layer characteristics and thesuppression of pore formation caused by the chlorine components presentin the treatment atmosphere, and the enhancement of boride formation, astwo reactions competing with one another, are determined by these andother plasma parameters. Depending on the pulse duty factor and the gascomposition it is possible, by adjusting the voltage in a specifiedmanner, to obtain a plasma condition characterized by a high particledensity of boron-releasing components, so that boride formationpreferentially takes place. Analysis of the plasma composition can becarried out, for example, with the aid of optical emission spectroscopy.In this connection, it has been found that the signals for the excitedboron, the excited BC1 and the C1 ⁺ signal, in particular, can be usedfor optimizing the layer characteristics. Those methods of carrying outthe invention in which the analysis shows high B signals were found tobe favorable. This is possible, for example, with voltages within anaverage range of preferably about 650 volts to about 800 volts, wherethe boron trihalide content of the gas mixture and the pulse-duty factorof the pulsed DC voltage play an additional role.

The method of the invention is suitable for industrial uses and can bedeveloped to a point where it is ready for series production. Comparedwith prior art boronizing methods of the kind discussed above, whichoperate with solid boron dispensing media, plasma boronizing with agaseous boron-releasing dispensing medium has an enormous improvementpotential. Handling of the components to be treated can be reduced to aminimum. The method of the invention is suitable for automation. Bychanging the treatment time, it is possible according to the method ofthe invention to change the gas composition so that an effect canthereby be exerted on layer formation, in which case special care shouldbe taken to avoid the formation of FeB. Furthermore, the methodaccording to the invention takes environmental considerations intoaccount since any residues of boronizing agent to be disposed of can beminimized.

Areas of industrial application of the method according to the inventionare, for example, boronizing of metal parts to increase the wearresistance of the surfaces of components which are exposed toparticularly high abrasive or adhesive stresses. The method according tothe invention is suitable, for example, for boride layer application tocomponents in the automobile industry, for example for gears, hydraulicplungers, camshafts, oil pump drives, for example, with crossed axes,helical gear wheels, and also for extruder screws and other componentsexposed to high stress.

The arrangement for the production of a boride layer on a surface byplasma boronizing according to the invention includes a reactor to whicha gas mixture containing a boron-releasing gas can be supplied and inwhich a glow discharge plasma is generated. The boronizing arrangementhas a plasma generator which furnishes a pulsed DC voltage with acontrollable pulse width and/or pulse pause.

1. The arrangement according to the invention preferably has at leastone mass flow meter for measurement and/or adjustment of the compositionof the gas mixture and/or the flow rate of one or more of the gases inthe gas mixture. In this way, the instantaneous composition of the gasmixture supplied to the reactor can be measured at any time, andthereupon the composition of the gas mixture and/or the supply of one ormore of the gases contained in the gas mixture can be changed. In thisway, it is possible to exert an influence on the process. For example,by changing the composition of the gas mixture during the process, thelayer formation may be altered, optionally as a function of theanalytical results of the determination of the particle composition inthe plasma. The method is preferably carried out with a gas mixturecontaining two or three components, for example, a boron trihalide,hydrogen and a noble gas. Hence, three mass flow meters are preferablyincluded, one for the measurement and/or adjustment of the supply ofeach of the three components, respectively,

Preferably a pressure gauge which is independent of the type of gas isused for measurement of the treatment pressure. The pressure gauge ispreferably computer-controlled.

The gases are distributed into the treatment chamber of the reactor, forexample, by a gas sprayer.

Furthermore, in the case of a thermally decomposable boron-releasinggas, it has been found advantageous to use a cooled gas inlet, since, inthat way, the boron-releasing gas can be more effectively utilized.

Moreover, in a further development of the invention, it is advantageousfor environmental reasons to use a gas purification device for treatmentof waste gas in order to minimize the amount of boron in the waste gasand thus minimize the environmental damage of the method. For thispurpose, for example, an arrangement in which the gas purificationdevice is attached to a vacuum pump for reducing the pressure in thetreatment chamber can be used can be used.

In a further embodiment of the invention, the reactor may contain anadditional heating device to achieve the desired treatment temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the inventions will be apparent from areading of the following description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a representative embodimentof an arrangement for producing a boride layer on a surface by plasmaboronizing according to the invention; and

FIG. 2 is a graphical representation showing the variation, as afunction of time, of the voltage for the pulsed DC voltage used in anembodiment of the method according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the schematic representation of a typical embodiment of anarrangement for carrying out the method of the invention for producing aboride layer on a surface by plasma boronizing shown in FIG. 1, areactor 10 has a treatment chamber 11 wherein a plasma is generated. Thetreatment chamber 11 of the reactor 10 is charged with a gas mixturecontaining a boron-releasing gas which is introduced into the treatmentchamber 11 through a gas inlet 12 and a supply line 13. Attached to thesupply line 13 are three feed lines through which the individualcomponents of the gas mixture are introduced. One of the components isthe boron trihalide, for example, boron trichloride or borontrifluoride, which is supplied through a branch line 14 which opens intothe supply line 13. The second component is hydrogen gas suppliedthrough a branch line 15 which likewise opens into the supply line 13.The third component is a noble gas, for example, argon, which is fedthrough a branch line 16 which also opens into the supply line 13. Threeflow meters 17, 18 and 19 are provided in the feed line 14, 15 and 16,respectively, of which the throughput of each of the components of thetreatment gas mixture can be adjusted and measured.

The reactor 10 further includes a charging plate 20 supported in thereactor chamber 11 on two rod insulators and a flow-guiding support (notshown). The voltage for generating the glow discharge is providedthrough a schematically shown voltage feed line 21. The plasma generatorprovides a pulsed DC voltage with a controllable pulse width and pulsepause, as explained hereinafter.

The composition and throughput of the treatment gas mixture are adjustedby controlling the mass flow meters 17, 18 and 19. The treatmentpressure is measured by a pressure gauge that is independent of the typeof gas and is controlled by a computer. The measurement of pressure andcontrol of the pressure are effected by a pressure control device 22which is connected to the treatment chamber 11 through a line 23.Connected to this line after the pressure control device 22 is a vacuumpump 24 and connected to the vacuum pump 24 in this line is a device 25for waste-gas purification, which provides for adequate waste-gastreatment as waste gases are withdrawn from the treatment chamber 11.

The temperature of the plasma generator is controlled by a temperaturecontrol device 26 and a control line 27.

Furthermore, the arrangement according to the invention has anadditional heating device 28 which is provided in the reactor 10 toestablish the desired treatment temperature in the treatment chamber 11.

The method of the invention for producing a boride layer preferably iscarried out at a low pressure, for example, in the range of 1 to 10 hPa,and is supported by electric activation of the gas atmosphere. Thecomponents to be boronized are cathodically connected to the containerwall of the treatment chamber. The gas mixture, preferably containing aboron trihalide, for example, boron trichloride or boron trifluoride,hydrogen and noble gas, is introduced into the treatment chamber 11 and,in addition to thermal activation, undergoes electric activation toproduce a through the glow discharge. The treatment temperature dependson the material of the respective components to be boronized and is, forexample, above 700° C., preferably 800° C. or higher.

A pulsed DC voltage is preferably applied in order to facilitateactivation of the surface by the noble gas/ion bombardment prior to thetreatment phase. Beyond that, during the treatment, active excited boronparticles are produced which reach the surface of the component wherethey primarily form borides by diffusion. The reduction of the halogenpresent in the atmosphere produced from the boron trihalide is enhancedby the atomic hydrogen produced in the plasma from the H₂ gas suppliedto the chamber.

The diagram of FIG. 2 shows, by way of example, a representative voltagevariation as a function of time for a pulsed direct current that isparticularly advantageous for a method according to the invention. Thevoltage represented is, for example, in a middle range at 650 volts,with the voltage pulse maintained for 160 μs, for example, and the pulsepause maintained for 50 μs, for example. Thus, the pulse pause isshorter by a factor of 3 than the duration of the DC voltage pulse. Thepulse period in this embodiment is 210 μs, and thus the frequency is4.762 kHz. In this embodiment, the pulse-duty factor, defined as theratio of the pulse duration to pulse pause within a pulse period is 3.2.It has been established that, when using a relatively high voltage, alonger pulse pause is required. However, when using argon in thetreatment gas, good results can be achieved even at relatively lowvoltages, for example, in the range above 500 volts. Although theinvention has been described herein with reference to specificembodiments, many modifications and variations therein will readilyoccur to those skilled in the art. Accordingly, all such variations andmodifications are included within the intended scope of the invention.

We claim:
 1. A method for producing a boride layer on a surface byplasma boronizing comprising the steps of: supplying a gas mixturecontaining a boron-releasing gas to a treatment chamber of a reactor;generating a glow discharge in the reactor; determining an amount of atleast one excited boron-releasing gas product selected from excitedboron and excited BC1 particles in the glow discharge; and selectingproduction parameters of the plasma generated in the treatment chamberof the reactor and one or more process parameters selected from at leastone of voltage, pulse duty factor, frequency, temperature and pressure,depending on the determined amount of the excited boron-releasing gasproduct so as to maintain at least one of at least one of a minimumvalue and a maximum value of the determined excited boron-releasing gasproduct, and at least one of a minimum value or a maximum value of arelation of one or more of the determined amount of the excitedboron-releasing gas product to another glow discharge product so as toproduce the boride layer on the surface.
 2. A method according to claim1, wherein said step of generating the glow discharge in the reactorcomprises using a pulsed DC voltage source having a ratio of voltagepulse duration to subsequent pulse pause duration which is greater than1.1:1.
 3. A method according to claim 1 wherein said step of generatingthe glow discharge in the reactor comprises applying a DC voltage inpulses having a pulse period of less than 230 μs.
 4. The methodaccording to claim 1, further comprising: during a first stage,generating the glow discharge in the reactor while maintaining the gasmixture at a selected treatment temperature to first produce said boridelayer and prevent formation of halogenides which cause formation ofpores, and during a second stage that is performed after the firststage, maintaining the gas mixture at a higher temperature than theselected temperature.
 5. A method according to claim 2 wherein the glowdischarge is generated by applying a DC voltage in pulses having a pulseperiod of less than 230 μs.
 6. A method according to claim 2 wherein themethod includes a first stage during which the gas mixture is maintainedat a selected temperature to prevent formation of halogenides whichcause formation of pores to first produce said boride layer, followed bya second stage during which the gas mixture is maintained at a highertemperature.
 7. A method according to claim 3 wherein the methodincludes a first stage during which the gas mixture is maintained at aselected temperature to prevent formation of halogenides which causeformation of pores to first produce said boride layer followed by asecond stage during which the gas mixture is maintained at a highertemperature.
 8. A method according to claim 1 including determining theamount of the excited boron-releasing gas in the reactor at least in arelative manner.
 9. A method according to claim 8 including determiningspectroscopically the amount of excited boron-releasing gas in thereactor.
 10. A method according to claim 8 including determining theamount of excited boron in the reactor at least as a function of theamount of excited boron-releasing gas in the reactor.
 11. A methodaccording to claim 1 wherein said supplied gas mixture comprises borontrihalide as the boron-releasing gas product in a concentration greaterthan about 1% by volume, along with hydrogen gas and, optionally, anoble gas.
 12. A method according to claim 4 wherein the glow dischargeis generated by applying a pulsed DC voltage which has a ratio of thevoltage pulse duration to the subsequent pulse pause duration in therange from about 1.1:1 to 5:1 ratio.
 13. A method according to claim 12wherein the ratio is in the range from about 1.5:1 to 3.5:1.
 14. Amethod according to claim 4 further comprising generating the glowdischarge using a pulsed DC voltage having a pulse period of less thanabout 210 μs.
 15. A method according to claim 14 wherein the pulsed DCvoltage has a pulse period ≧50 μs.
 16. A method according to claim 15wherein the voltage of the pulsed DC voltage used for generating theflow discharge in the range between about 500 volts and about 1000volts.
 17. A method according to claim 16 wherein the pulsed DC voltageis in the range between about 650 volts and about 800 volts.
 18. Amethod according to claim 1 wherein the reactor pressure is maintainedin a pressure range between about 0.5 and about 15 hPa.
 19. A methodaccording to claim 18 wherein the reactor pressure is maintained in therange between about 1 and about 10 hPa.
 20. A method according to claim1 wherein the gas mixture contains a boron trihalide in a concentrationof between 2% by volume and about 50% by volume.
 21. A method accordingto claim 20 wherein the boron trihalide concentration is between about2% by volume and about 10% by volume.
 22. A method according to claim 1wherein the gas mixture contains up to 20% by volume of a noble gas and2% by volume to 50% by volume of boron trihalide, the remainder beinghydrogen gas.
 23. A method according to claim 1 wherein the gas mixturecontains more than 0% and up to 20% by volume of argon and 2% by volumeto 50% by volume of boron trihalide, and wherein the remainder of thegas mixture is a hydrogen gas.
 24. A method according to claim 22wherein the gas mixture contains 2% by volume to 20% by volume of borontrihalide.
 25. A method according to claim 1 wherein the boron-releasinggas is one of BCl₃, BF₃ and mixtures thereof.
 26. A method for producinga boride layer on a surface by plasma boronizing comprising the stepsof: supplying a gas mixture containing a boron-releasing gas to atreatment chamber of a reactor; generating a glow discharge in thereactor; determining a first amount of at least one excitedboron-releasing gas product selected from excited boron and excited BC1particles in the glow discharge; selecting first values for productionparameters of the plasma generated in the treatment chamber of thereactor and one or more process parameters selected from at least one ofvoltage, pulse duty factor, frequency temperature and pressure,depending on the first determined amount of the excited boron-releasinggas product so as to maintain at least one of at least one of a minimumvalue and a maximum value of the excited boron-releasing gas product,and at least one of a minimum value or a maximum value of a relation ofone or more of the amount of the first determined excitedboron-releasing gas product to another glow discharge product to producethe boride layer on the surface; determining a second amount of at leastone excited boron-releasing gas product in the glow discharge; andreturning to the selecting step to be performed using selecting secondvalues instead of the first values.